1. Field of the Invention
The present invention relates to an analyzer for a trace biosubstance and, more particularly, to a mass spectrometer suitable for proteomics which analyzes proteins in a comprehensive manner.
2. Description of Related Art
Heretofore, in high-sensitivity mass spectrometry for trace biosubstances, there widely has been used an electrospray ionization (ESI) mass spectrometry. The details of ESI which produces gaseous ions is described in Science, Vol. 246, pp. 64–71, 1989. In the conventional ESI, a sample solution is introduced into a metallic capillary about 0.2 mm in outside diameter and a high electric field is applied to the sample solution at an end portion of the capillary. As a result, with the high electric field, the sample solution is withdrawn from the capillary end portion and a liquid cone is formed. At a tip portion of the cone, ions of the same polarity are concentrated, so that a repulsive force between ions increases to a level equal to the surface tension of liquid and charged droplets are discharged from the cone tip which has become unstable. The charged droplets thus produced evaporate and release gaseous ions. The gaseous ions thus generated are introduced into a vacuum device and are analyzed by means of a mass spectrometer.
Further, as described in Book of Abstracts, Annual Conference on Mass Spectrometry, the Mass Spectrometry Society of Japan, pp. 36–37 (1995), there has been proposed a structure in which a central axis of a capillary and that of an ion intake port in a mass spectrometer are made substantially orthogonal to each other. According to this technique, it is possible to somewhat eliminate charged particles and introduce only gaseous ions preferentially into a vacuum device.
Generally, in ESI, the ion producing efficiency tends to become higher as the flow rate of a sample solution decreases. However, if the flow rate of a sample solution is not higher than 1 μL/min (microliter/min) the evaporation of solvent from a liquid cone becomes too high, with the result that the production of ions becomes unstable or the ion producing efficiency becomes lower with the lapse of time. In view of this point, there has been developed a nanospray chip made of quartz wherein only a capillary end is formed as small as several μm to 10 μm. In this miniaturized ESI, since the solvent evaporation effect becomes lower, ions can be produced stably in such an extremely low flow rate range of a sample solution from 1 μL/min to 1 nL/min (nanoliter/min).
Moreover, since the flow rate of a sample solution is low, the size of the resulting charged droplet also becomes small, with consequent improvement of the ion producing efficiency. For this reason, a nanospray is often used at present for protein analysis. In many cases, a central axis of a capillary is aligned with that of an ion intake port in a mass spectrometer.
On the other hand, as an extremely soft ionization method there has been developed a sonic spray ionization method (SSI) which produces gaseous ions by spraying a sample solution together with a high-speed current of gas, e.g. sonic gas current, from a capillary end as described in U.S. Pat. No. 6,147,347 and U.S. Pat. No. 6,114,693. According to SSI, with a shear force induced by a sonic gas current, charged fine droplets are produced from a sample solution and gaseous ions are generated efficiently. The ion producing efficiency tends to increase with a decrease of liquid flow rate.
In SSI, however, a quartz capillary having an outside diameter of about 200 μm and a flow rate of above 10 μL/min have so far been used in many cases. This is because if the capillary is used at a flow rate of below 10 μL/min; the suction of liquid by a sonic gas current becomes too high at the capillary end and it becomes difficult to stabilize the production of ions. If the flow velocity of gas is low, the liquid suction effect becomes low, but the size of a droplet formed by spray becomes large and, therefore, the ionization efficiency is not high.
In the case where a mixed solution containing trace biosubstances extracted from a living body is separated by liquid chromatography (LC), the liquid flow rate is lower and the separation is expected to be higher. For this reason, in a liquid chromatography/mass spectrometry (LC/MS) system it is desirable to decrease the liquid flow rate in LC. In LC/MS interface or ion producing section, the ion producing efficiency tends to becomes higher as the liquid flow rate becomes lower. Therefore, decreasing the liquid flow rate is important in high-sensitivity analysis of trace biosubstances.
A non-volatile substance comprising an impurity is certain to be mixed in a charged droplet produced by spray. Therefore, after evaporation of a volatile solvent, the charged droplet remains as a charged particle. If this charged particle is introduced, together with ion, into a vacuum device, not only is the mass spectrometer contaminated, but also it becomes a noise source in ion detection, thus making peak determination difficult.